This paper describes the results of a short-term experiment using measurements of s_v from the diel, spatial, clupeoid distribution in the southern Baltic. The aim of the experiment was to verify fish-behaviour characteristics measured over the period from 1995 to 2001. It was also intended to estimate the dynamics of fish behaviour over one separate diel cycle. The studies were based on a 24 h continuous integration of fish echoes using an EY500 echosounder at 38 kHz. Measurements were carried out by RV “Baltica” travelling along the sides of a square of 4 nautical miles at a constant speed...

This paper describes the results of a short-term experiment using measurements of s_v from the diel, spatial, clupeoid distribution in the southern Baltic. The aim of the experiment was to verify fish-behaviour characteristics measured over the period from 1995 to 2001. It was also intended to estimate the dynamics of fish behaviour over one separate diel cycle. The studies were based on a 24 h continuous integration of fish echoes using an EY500 echosounder at 38 kHz. Measurements were carried out by RV “Baltica” travelling along the sides of a square of 4 nautical miles at a constant speed of 8 knots. The South Gotland Deep was chosen for the experiment because of the greatest amplitude there of fish vertical, diel migrations within the Polish EEZ. The duration of the experiment was limited by weather conditions, but nearly 300 EDSU samples were collected. 3-D distributions of echoes were correlated to the values of coincident environmental factors of time, depth, water temperature, salinity, and oxygen level. Fish distribution compared with environmental factors is described by different macrosounding visualizations, statistical, and mathematical models. Measurements are compared with the average characteristics of fish behaviour based on data from the autumn acoustic studies between 1995 and 2001 in a wider environment of the Gotland Deep. The results strongly confirmed the instability of the diel acoustic response of fish echoes in both situations. More precise measurements during the experiment indicated the biggest column-scattering strength (S_vc) increase during the sunrise period, appearing simultaneously in the whole area. A similar increase was detected by analysis of 1995–2001 data from the South Gotland Deep environment. Major emphasis is given to the explanation of the diel irregularities. Diel instability of fish acoustic response can significantly effect the results of target-strength measurements (up to 400% during the sunrise) and, as a consequence, the calibration of acoustic fish stock-assessment models.